A marine researcher says coral bleaching near Lizard Island is at its worst in 30 years.

Supplied: Professor Justin Marshall

The coral bleaching threat level on the Great Barrier Reef Marine Park is expected to increase next week, with marine experts saying it is already at its worst.

University of Queensland marine researcher Professor Justin Marshall, who has been visiting the reef for 30 years, said the bleaching in the northern section of the World Heritage Area was the worst he had ever seen.

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